1. Field of the Invention
This invention relates to high definition television. More particularly, this invention relates to a method and apparatus for encoding video signals to fit within a limited transmission capacity.
2. Description of the Prior Art
Video signals typically originate from video cameras. The bandwidth of video signals is quite substantial and, consequently, practitioners in the art have tried to reduce the bandwidth of these signals without unduly degrading the images. Typically, to reduce bandwidth, the video signals are encoded, and redundancies in the encoded signals are extracted and deleted. Different techniques are used in the art. Some are better suited for still images, while others are better suited for moving images. One of the techniques for reducing the bandwidth of moving images is generally referred to as motion compensated predictive coding.
In conventional motion compensated predictive coding, each video frame is first partitioned into square blocks of picture elements (pels); such as blocks of 8 pels by 8 pels. Each block is coded, in turn, and the developed encoded sequence is transmitted over a communications channel to a decoder. The communications channel may be, or may include, a storage element. Following the partitioning step in the encoding process, a determination is made as to whether or not the pels of the block have changed significantly compared with the previous frame. If not, an indicator signal is sent which signifies to the decoder that it needs to merely repeat the pels of that block from the previous frame to obtain the pels for the current block. This step is known as "Conditional Replenishment". If the pels have changed since the previous frame, an attempt is made to determine the best estimate of motion that is occurring in the block. This is frequently done by a "Block Matching Motion Estimation" technique wherein the pels of the current block are successively compared with various small shifts of the corresponding block in the previous frame. The shift that gives the best match is deemed to be the "best estimate" of the displacement in the block's image between frames, and the amount of this shift, called the "Motion Vector", is selected and sent to the decoder.
The "best estimate" is, of course, the estimate that yields the smallest difference signal between the image in the current block and the image in the shifted block of the previous frame. This difference signal forms the error signal. When the error signal is sufficiently small, an indicator signal is sent to the decoder, which merely causes the pels of the shifted block from the previous frame to be repeated for the pels for the current shifted block. Such blocks are said to have been successfully "Motion Compensated". However, if there is a significant difference between the two blocks, the difference is encoded and sent to the decoder so that the pels of the current block may be more accurately recovered. Coding of this difference is typically performed by means of the "Discrete Cosine Transform" (DCT). It is a measure of energy.
The amount of coded information that is generated by the above procedure is variable. It can be appreciated, for example, that image changes that do not correspond to a uniform translation, or motion, of the image may require substantial encoding to describe the deviation of a block from its best translated replica. On the other hand, when the image does not change between successive frames, then there is a minimal amount of information that needs to be encoded. To accommodate these potentially wide variations in the amount of code that needs to be transmitted, typical encoders include a memory at the output, to serve as a buffer.
The buffer is not a panacea, however. For a given transmission rate, when an excessive volume of data is generated, there is always a danger that the FIFO would overflow. When it does, coding must stop until the transmission channel can empty the FIFO sufficiently to permit new data to be inserted.
All of the above teachings in the art deal with the coding and decoding aspects of reducing the bandwidth of the TV signal, but none deal explicitly with the formatting of the signal in preparation for transmission. When it comes to high definition television, both the bandwidth and the formatting problems must be solved and the difficulties are even greater than in connection with conventional TV signals because the desired signal compression is even greater, and because the requirement for a more authentic representation of the original image are more stringent.